You could possibly have a bad coil that is arcing internally or you could have a clogged injector that is causing that cylinder to run a bit lean. Did you check the condition of the coil terminals to the spark plug wires when you replaced them? It is very common to see rust or corrosion here and it can definately cause a misfire. An easy way to check out the injectors is to go around to each one with a stethoscope and listen to the sound they make. I have an injector pulse tool that is purpose meant to check the spray pattern and output of the fuel injectors but is not something that is too easy to come across and requires removing the fuel rail from the car. A compression test will also verify that the cylinder is mechanically sound or at least is close to the other 5 if it is then you are left with fuel and spark.
